The bgp advertises network 205.1.2.0 255.255.255.0 and it also has ip route 205.1.2.0 255.255.255.0 Null0. It is said that the static route is used for loop prevention. But I don't sget it. I advertise my 205.1.2.0/24 to my bgp neighbor and at the same time I route it to a null0.

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For a route to be advertised via BGP, it must first exist in the routing table via a directly connected interface, a static route, ospf, isis, etc.

Typically, supernets are nailed down in the routing table via a Null0 static route.

Since this is only a /24, I would expect that it's already in the routing table via one of the options above. If this is one of several subnets and you want to make sure that the prefix isn't dropped via BGP, leaving it Null routed is fine but I typically put weights are Null routed statics when using them for inejction into the routing table, i.e.,

ip route 205.1.2.0 255.255.255.0 Null0 250

When you look at your running configuration, is this entire subnet already being routed or connected?

No, you wouldn't put an entire /24 on a loopback interface just to inject it into the routing table. That would be a complete waste.

Biggynet: If you do have not a downstream device propagating that route via OSPF, IS-IS or EIGRP, then that Null static route will take care of the problem. However, you would be better served to weight that route (put 250 after the 'null0').

Presumably you are using pieces of this /24 in your network. As previusly mentioned, you need to have an exact match in your IGP routing table in order for BGP to advertise that network. The static route to null0 provides it.

Remember that your router will always route to the most specific route, so if you already have a route to some subnet of the /24, the router still follows it. You don't have to worry about changing the AD (I think the previous poster actually meant AD, not weight) of the /24 route to null0 because it will only be used for addresses in that /24 for which there is no other route.

So if I understand correctly, When I do:
"router bgp 9999
network 201.1.1.0 mask 255.255.255.0
neighbor 205.1.1.1 remote as 1111"

Nothing will be advertised to my peer if I don't have a route in my routing table. But if I use ip route 201.1.1.0 255.255.255.0 null0. My router will then advertise the network 205.1.1.0 to my peer. Correct?

Adding on what Mike said, the null 0 route is used to advertise a supernet or parent block, in this case it is a /24. This prefix(/24) is not used or assigned to any network but the smaller parts of the subnet are used, or the /24 was subnetted. Like Mike said, this works because traffic is routed to your site because of the /24 advertisement but once it is routed to the advertising router the router has a more specific route to the other portions of that subnet that were subnetted, maybe 16 /28s (16 /28s = 1 /24). If there are more than one route for a network the longest match is the preferred route.

Why is this done? to keep the global routing table as small as possible and to minimize the number of route advertisements network engineer has to make, otherwise instead of advertising the one route (/24) we potentially would need to advertise 16 /28s, you see how this could dramatically increase the routing tables of internet routers?

It is for efficient global routing table management as well as a best current practice.

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